Japan yesterday rejected a U.S. request that it join a trade embargo against Iran, but agreed to urge Japanese companies not to undercut the boycott with any increased purchases of Iranian oil.

Deputy Foreign Minister Ryohei Murata delivered Japan's response to Undersecretary of State Michael H. Armacost, who is visiting Tokyo. Japan relies on the Persian Gulf for a major percentage of its crude oil supplies and expresses a vital interest in maintaining a neutral position in the Iran-Iraq war.

A Japanese Foreign Ministry communique quoted Murata as saying that "Japan understands the circumstances that led the United States to impose the economic sanctions. However, Japan finds it difficult to take similar measures."

A Japanese official in Tokyo expressed confidence that Japanese companies, stung by criticism when they stepped up their commerce with Iran during the U.S. hostage crisis, will follow government policy.

"They won't want to repeat that kind of stupid action, particularly against the background of severe economic tension between the two countries," the official said, referring to trade tensions between the United States and Japan. "If they behave rationally, they will not want to make themselves targets of anti-Toshiba-like criticism."

Japan's relations with Washington plummeted earlier this year when it was revealed that Toshiba Machine Co. had sold the Soviet Union militarily useful machine tools. Partly because of the Toshiba scandal, officials here were eager to find some way to cooperate with President Reagan's call last week for all U.S. allies to boycott Iranian oil in response to what Reagan called Iran's increasing belligerence in the Persian Gulf war.

A senior Soviet official arrived in Iran as part of a tour of the region aimed at moving Iran and Iraq toward acceptance of a three-month-old United Nations cease-fire resolution. First Deputy Foreign Minister Yuli Vorontsov said Moscow was "keen to be informed" about Iran's current view of the truce call, according to news services quoting the Iranian state news agency.

Vorontsov had earlier told officials in Kuwait that Moscow has pressed Iran to halt missile attacks on Kuwaiti installations, Kuwait's official news agency said.

Iraq claimed its jets hit a large vessel in the gulf, and shipping sources said fire had broken out anew on an Iranian platform shelled Oct. 19 by the U.S. Navy in retaliation for an Iranian missile attack on a U.S.-flagged Kuwaiti tanker. The sources told The Associated Press that fires have erupted periodically on the platform since the attack. Washington Post correspondents Fred Hiatt and Margaret Shapiro in Tokyo contributed to this report.